Method for producing monodisperse bubbles

ABSTRACT

The invention provides a method for producing bubbles that exhibit an excellent monodispersity. The invention relates to a method for generating bubbles by the injection and dispersion of a gas through a porous body into a liquid, wherein the value produced by dividing the pore diameter that accounts for 10% of the total pore volume in the relative cumulative pore distribution curve of the porous body by the pore diameter that accounts for 90% of the total pore volume in the relative cumulative pore dismeter distribution curve of the porous body is 1 to 1.5.

TECHNICAL FIELD

The present invention relates to a method for producing monodispersebubbles.

BACKGROUND ART

Various methods for generating bubbles have already been proposed.Examples in this regard are a) gas transport methods in which a gas ispassed through the micropores of a gas dispersing tube into a liquid; b)methods in which a vibration with a frequency no greater than 1 kHz isapplied to a porous body while a gas is being fed into a liquid throughthe porous body; c) bubble generation methods that utilize ultrasound;d) shaking•stirring methods in which bubbles are generated by stirring aliquid and shearing a gas; e) methods in which a gas is dissolved underpressure in a liquid followed by pressure reduction in order to generatebubbles from the supersaturated dissolved gas; and f) chemical foamingmethods in which bubbles are created by generating a gas in a liquid bya chemical reaction (refer, for example, to Clift, R. et al., “Bubbles,Drops, and Particles”, Academic Press (1978), and Hideki TAKUSHOKU,“Progress in Chemical Engineering. 16. Bubble, Drop, and DispersionEngineering”, Maki Shoten, 1 (1982)).

However, these methods, excluding methods that generate microfinebubbles utilizing microwaves, not only have difficulty producing veryfine bubbles with bubble diameters on the order of nanometers, but alsosuffer from the problem of an impaired stability due to a nonuniformbubble diameter. In addition, it is also extremely difficult in theaforementioned methods to freely adjust the bubble diameter.

DISCLOSURE OF THE INVENTION

A main object of this invention is to provide a method for generatingbubbles that exhibit an excellent monodispersity.

As a result of extensive and focused investigations, the inventordiscovered that this object could be achieved by applying pressure to agas and dispersing it into a liquid through a special porous body. Thisinvention was achieved based on this discovery.

That is, the present invention relates to the following method forpreparing bubbles.

1. A method for producing bubbles by the injection and dispersion of agas through a porous body into a liquid,

wherein the porous body has a value of 1 to 1.5,

wherein the value is given by dividing the pore diameter that accountsfor 10% of the total pore volume in the relative cumulative poredistribution curve of the porous body by the pore diameter that accountsfor 90% of the total pore volume in the relative cumulative porediameter distribution curve of the porous body.

2. The method according to above 1, wherein the contact angle withrespect to the liquid of at least the surface of the porous body that isin contact with the liquid is greater than 0° and less than 90°.

3. The method according to above 1, wherein porous glass is used as theporous body.

4. The method according to above 1, wherein the liquid contains at leastone additive selected from the group consisting of emulsifying agents,emulsion stabilizers, foaming agents, and alcohols.

5. Bubbles obtained by the method according to above 1.

6. The bubbles according to above 5, wherein, in the integrated volumedistribution of the bubbles,

1) the diameter at which the bubble volume accounts for 10% of the totalbubble volume is at least 0.5-times the diameter at which the bubblevolume accounts for 50% of the total bubble volume, and

2) the diameter at which the bubble volume accounts for 90% of the totalbubble volume is no more than 1.5-times the diameter at which the bubblevolume accounts for 50% of the total bubble volume.

ADVANTAGES OF THE INVENTION

The method according to the present invention can reliably producehighly monodisperse bubbles. The method according to the presentinvention in particular can also provide microfine monodisperse bubblesfor which the bubble diameter size is in the nanometer range(monodisperse nanobubbles). In addition, the method according to thepresent invention also enables the bubble diameter to be freely adjustedby varying, for example, the pore diameter of the porous body.

The monodisperse bubbles and particularly the nanobubbles and/ormicrobubbles (microfine monodisperse bubbles for which the bubblediameter size is in the micrometer range) obtained by the methodaccording to the present invention can be used in a broad range offields, such as hydroponic cultivation, the cultivation of marineproducts, bubble-containing food products, microcapsules, pharmaceuticalpreparations and cosmetics, various foam materials, and separationprocesses such as ore flotation and bubble-utilizing foam separation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram that shows an example of an apparatus forexecuting the method according to the present invention.

FIG. 2 is a schematic diagram of a bubble-generating apparatus.

FIG. 3 shows the bubble diameter distribution of the nanobubblesobtained in Example 1.

FIG. 4 shows the relationship between the average pore diameter of aporous glass membrane and the average bubble diameter.

FIG. 5 shows the relationship between the critical pressure and theaverage pore diameter of a porous glass membrane.

BEST MODE FOR CARRYING OUT THE INVENTION

The method according to the present invention for producing bubbles is amethod for producing bubbles by the injection and dispersion of a gasthrough a porous body into a liquid,

wherein the porous body has a value of 1 to 1.5,

wherein the value is given by dividing the pore diameter that accountsfor 10% of the total pore volume in the relative cumulative poredistribution curve of the porous body by the pore diameter that accountsfor 90% of the total pore volume in the relative cumulative porediameter distribution curve of the porous body.

As used hereinbelow with reference to the present invention, the “10%diameter” refers to the pore diameter that accounts for 10% of the totalpore volume in the relative cumulative pore distribution curve of theporous body while the “90% diameter” refers to the pore diameter thataccounts for 90% of the total pore volume in the relative cumulativepore diameter distribution curve of the porous body.

The Porous Body

The porous body used by the method according to the present inventionhas a relative cumulative pore diameter distribution curve in which thevalue given by dividing the 10% diameter by the 90% diameter is 1 to 1.5and preferably 1.2 to 1.4. The use of a porous body having a porediameter distribution in this range (that is, a porous body with auniform pore diameter) enables the reliable production of bubbles thatexhibit an excellent monodispersity.

The pore diameter of the porous is not specifically restricted, but cangenerally be set upon as appropriate from within the average porediameter range of 0.02 to 25 μm (preferably 0.05 to 20 μm). The averagebubble diameter of the monodisperse bubbles can also be freely adjustedin particular within the range of about 0.2 to 200 μm by adjusting thepore diameter.

The porous body can be any porous body that has a uniform pore diameteras defined hereinabove. The pore shape is not particularly limited aslong as the pore shape is that of a through pore, and the pore shape canbe exemplified by a cylindrical column, a square column, and so forth.The pores can run through perpendicular to the surface of the porousbody or can run through obliquely, and the pores can be intertwined witheach other. The pores in the porous body preferably have a uniformhydraulic diameter. Such a pore structure is very suitable for use bythis invention.

The shape of the porous body is also not limited and may be any shapecapable of dispersing a gas into a liquid. The porous body can be, forexample, membrane shaped, block shaped, disk shaped, square columnshaped, cylindrical column shaped, and so forth. This can be selected asappropriate in accordance with the intended use, service, and so forth.A membrane-shaped porous body can generally be suitably used. Amembrane-shaped porous body can have the shape of a flat membrane or apipe. In addition, a membrane-shaped porous body can be a symmetricmembrane or an asymmetric membrane. Moreover, a membrane-shaped porousbody can be a uniform or nonuniform membrane. These shapes andstructures are selected as appropriate in correspondence to the type ofliquid used, the intended bubbles, and so forth.

The size of the porous body is also not limited and can be selected asappropriate in view of the bubble generation application, the method ofusing the porous body, and so forth.

The material constituting the porous body is also not limited and can beselected as appropriate. Preferred materials can be exemplified byglasses, ceramics, silicon, polymers, or the like. Glasses (porousglasses) in particular can be suitably used by the present invention.Suitable for use as the porous glass is, for example, porous glassproduced utilizing microphase separation in glass. The known porousglasses can be used as such porous glass, and, for example, porousglasses produced utilizing microphase separation in glass can besuitably used. Specific examples are the CaO—B2O3-SiO2-Al2O3-basedporous glass disclosed in Japanese Patent 1,504,002 and theCaO—B2O3-SiO2-Al2O3-NaO2-based porous glass andCaO—B2O3-SiO2-Al2O3-NaO2-MgO-based porous glass disclosed in JapanesePatent 1,518,989 and U.S. Pat. No. 4,657,875. Also usable is theSiO2-ZrO2-Al2O3-B2O3-NaO2-CaO-based porous glass disclosed in JapanesePublished Patent Application No. 2002-160941.

The porous body in the present invention desirably exhibits good wettingby the liquid used. Porous bodies that are either poorly wetted or notwetted by the liquid used can also be used after execution thereon of asurface treatment or surface modification by a known method so as to bewettable by the liquid used. Wetting by the liquid denotes a contactangle by the liquid on the surface of the porous body preferably greaterthan 0° and less than 90°, particularly preferably greater than 0° andless than 45°, and more preferably greater than 0° and no greater than30°.

The Gas

There are no particular limitations on the gas used by the presentinvention, and a desired gas can be used as appropriate. The gas used bythe present invention can be exemplified by at least one selection fromthe group consisting of substances that are gases at ambienttemperature, such as air, nitrogen gas, oxygen gas, ozone gas, carbondioxide, methane, hydrogen gas, ammonia, and hydrogen sulfide, and thevapors of substances that are liquid at ambient temperature, such asethyl alcohol, water, and hexane.

The Liquid

There are also no particular restrictions on the liquid used by thepresent invention, and a variety of liquids can be used. The liquid usedby the present invention can be exemplified by water and by oil-miscibleliquids such as oils, fats, and organic solvents.

An additive can also be added to the liquid in the present invention inorder to stabilize the obtained bubbles. Preferred for use as theadditive is at least one selection from emulsifying agents, emulsionstabilizers, foaming agents, and alcohols.

The emulsifying agent can be any emulsifying agent that has the abilityto lower the interfacial tension of the liquid, and known emulsifyingagents and commercial products can be used. In addition, either awater-soluble emulsifying agent or an oily emulsifying agent can be usedas the emulsifying agent.

The known hydrophilic emulsifying agents can be used as thewater-soluble emulsifying agent. For example, nonionic emulsifyingagents can be exemplified by glycerol fatty acid esters, sucrose fattyacid esters, sorbitan fatty acid esters, polyglycerol fatty acid esters,polyoxyethylene hydrogenated castor oil,polyoxyethylene-polyoxypropylene glycols, lecithin, and polymericemulsifying agents. The anionic emulsifying agents can be exemplified bycarboxylic acid salts, sulfonic acid salts, and sulfate ester salts. TheHLB of these hydrophilic emulsifying agents is preferably at least 8.0and more preferably is at least 10.0 These hydrophilic emulsifyingagents can be used individually or in combinations of two or more incorrespondence to the desired emulsifying activity. The quantity ofaddition of these hydrophilic emulsifying agents is not specificallylimited as long as an adequate emulsifying effect is obtained;generally, however, about 0.05 to 1 weight % with reference to theemulsion as a whole will be appropriate.

Nonionic emulsifying agents, for example, can be used as the oilyemulsifying agent. More specific examples are glycerol fatty acidesters, sucrose fatty acid esters, sorbitan fatty acid esters, propyleneglycol fatty acid esters, polyglycerol fatty acid esters,polyoxyethylene hydrogenated castor oil,polyoxyethylene-polyoxypropylene glycols, lecithin, and so forth. Thesecan be used individually or two or more can be used. Particularlypreferred among the preceding are polyglycerol fatty acid esters,sucrose fatty acid esters, and so forth. The quantity of addition of theoily emulsifying agent can be determined as appropriate in view, interalia, of the type of oily emulsifying agent used; generally, however,about 0.05 to 30 weight % in the liquid is appropriate.

The emulsion stabilizer is a substance that coats the gas-liquidinterface of the generated bubbles and thereby stabilizes the bubbles.The emulsion stabilizer can be exemplified by synthetic polymers such aspolyvinyl alcohol and polyethylene glycol. Its quantity of addition isnot particularly limited as long as a satisfactory bubble-generatingeffect is obtained; generally, however, about 0.05 to 50 weight % in theliquid is appropriate.

The foaming agent is a substance that can facilitate bubble generation,but is not otherwise limited. The foaming agent can be exemplified byglycosides such as saponins; polysaccharides such as sodium alginate andcarrageenan; and proteins such as albumin and casein. The quantity ofaddition is not limited as long as a satisfactory bubble-generatingeffect is obtained; generally, however, about 0.05 to 50 weight % in theliquid is appropriate.

The alcohol can be exemplified by ethyl alcohol, propyl alcohol, andbutanol. Addition of the alcohol facilitates bubble generation byreducing the interfacial tension γ of the liquid. The quantity ofalcohol addition is not particularly limited as long as an adequatebubble-generating effect is obtained; generally, however, about 0.05 to50 weight % in the liquid is appropriate.

The method for generating monodisperse bubbles

The method according to the present invention generates bubbles by theinjection and dispersion of a gas through the porous body describedhereinabove into a liquid.

There are no particular limitations on the procedure for injection anddispersion. Injection and dispersion can be carried out, for example, asfollows. First, a side of the porous body is brought into contact with aliquid and another side is brought into contact with a gas. Then, bypressurizing the gas, the gas is caused to traverse the through pores ofthe porous body and to disperse into the liquid. Methods forpressurizing the gas can be exemplified by methods in which the gas isforcibly filled into a sealed space and methods in which the gas isfilled into a sealed space and the air is thereafter compressed with,for example, a piston.

An example of a preferred embodiment of the execution of the methodaccording to the present invention is provided hereafter. A liquid (c)is transported to a porous glass membrane and membrane module (a) by apump (d). A gas in a gas cylinder (b) is transported to the porous glassmembrane and membrane module (a) under regulation by a valve (e) whilereferring to a pressure gauge (f). Proceeding in this manner enables thedispersion of bubbles in the liquid. The particle diameters of theobtained bubbles can be measured by a particle size distributionanalyzer based on the laser diffraction method (g).

FIG. 2 is a schematic diagram of bubble generation at the porous bodywhen the gas is pressurized. The minimum pressure ΔPc at which bubblegeneration begins is generally given by the following equation;ΔP=4γ cos θ/Dm

wherein γ is the surface tension of the liquid relative to the gas, θ isthe angle of contact relative to the air of the liquid present at thesurface of the porous body, and Dm is the average pore diameter of theporous body.

In the present invention, in order to obtain monodisperse bubbles havinga smaller average bubble diameter, the pressure difference ΔP (=PA−PL)between PA of the gas when the gas is pressurized and the pressure PL ofthe liquid is desirably controlled to about 0.2 to 10 MPa andparticularly about 1 to 5 MPa.

Bubble generation may be carried out by the present invention accordingto either a batch or continuous regime. The continuous regime, whenused, is desirably carried out as follows. When, for example, the porousbody is a flat membrane, the liquid is preferably stirred with, forexample, a stirrer. When, for example, the porous body is a tubularmembrane, the liquid is preferably circulated using a pump. The particlediameter of the obtained monodisperse bubbles can be measured by knownmethods using commercially available particle diameter measurementinstruments.

The Bubbles

The bubbles obtained by the method according to the present invention(bubbles according to the present invention) in general have smallbubble diameters and are monodisperse. In particular, the bubbles have ahigh monodispersity that, in the cumulative volume distribution of thebubbles, the diameter at which the bubble volume accounts for 10% of thetotal bubble volume is at least 0.5-times (preferably about 0.6- to0.8-times) the diameter at which the bubble volume accounts for 50% andthe diameter at which the bubble volume accounts for 90% of the totalbubble volume is no more than 1.5-times (preferably about 0.2- to1.4-times) the diameter at which the bubble volume accounts for 50%.

While there is no limitation on the average bubble diameter of thebubbles according to the present invention, this value is ordinarilyabout 0.2 to 200 μm and can be decided upon as appropriate incorrespondence to the specific application and so forth. In particular,the bubble diameter of the bubbles can be controlled into a freelyselected range in the method according to the present invention byaltering the pore diameter of the porous body used. The method accordingto the present invention can also produce, for example, 400 nm to 900 nmnanobubbles.

The bubbles according to the present invention can be used in a varietyof applications, such as in the medical field and for agriculturalchemicals, cosmetics, food products, and so forth. With regard tomedical applications, the bubbles according to the present invention canspecifically be used in contrast media and drug delivery system (DDS)formulations. When nanobubbles are incorporated into the contrast mediaused in ultrasound diagnosis, the sensitivity of the contrast media isdramatically improved due to the fact that the bubbles exhibit a uniquesensitization action with respect to ultrasound. In addition, theintroduction of bubbles into microcapsules also makes it possible torupture the microcapsules at a target region by exposure to shock wavesand thereby release a drug present in the capsule.

In the field of food products, the stability of the monodispersenanobubbles or monodisperse microbubbles can be used to improve thetexture and taste of, for example, mousse food products. In addition, byinjecting nanobubbles of an inert gas such as nitrogen into a beverage,such as milk or PET bottle or bag tea, the dissolved oxygen that is acause of beverage deterioration can be very efficiently removed, therebyenabling an inhibition of quality deterioration.

With regard to cosmetic applications, the stability of the monodispersenanobubbles or monodisperse microbubbles enables use as a high-qualitymousse (hair setting materials, skin cream, and so forth).

With regard to biological and chemical applications, the invention canbe very suitably used in hydroponic cultivation, marine cultivation, andso forth, by utilizing the very large surface area of nanobubbles andmicrobubbles for the dissolution of oxygen in water. In addition, watercan also be sterilized very efficiently using ozone nanobubbles.Moreover, because nanobubbles and microbubbles exhibit a bindingactivity for substances present in the liquid, due to their largesurface area they can very efficiently inhibit the proliferation ofmicroorganisms (antimicrobial activity) and can very efficiently effectthe separation and recovery of suspended material (ore flotation andfoam separation).

Otherwise, bringing the body into contact with nanobubbles ormicrobubbles at, for example, a bathhouse or hot spring, provides betterstimulation of blood flow, a better temperature maintenance effect, abetter skin reviving effect, and so forth.

EXAMPLES

The invention is described in additional detail hereinbelow throughexamples. However, the scope of the invention is not limited to theseexamples.

Example 1

Using the apparatus shown in FIG. 1, air was injected and dispersedthrough a tubular porous glass membrane having an average pore diameterof 85 nm (SPG membrane from SPG Technology Co., Ltd.) into an aqueoussolution containing 0.1 weight % anionic emulsifying agent (sodiumdodecyl sulfate). The pressure difference ΔP between the air and theaqueous solution was 3.0 MPa and the liquid temperature was 25° C. Theaqueous solution was transported by a pump and the in-tube flow velocitywithin the membrane was set at 4.0 m/s.

The generated bubbles were directly introduced into the measurement cellof a particle diameter distribution measurement instrument (productname: “SALD2000”, from the Shimadzu Corporation). The obtained bubblediameter distribution is shown in FIG. 3. As is clear from FIG. 3, theobtained bubbles were highly monodisperse nanobubbles having an averagebubble diameter of 750 nm.

Example 2

The relationship between the pore diameter of the porous glass membraneand the average bubble diameter of the generated bubbles wasinvestigated in accordance with Example 1 by varying the average porediameter of the porous glass membrane. The results are shown in FIG. 4.As is clear from FIG. 4, a linear relationship given by Dp=8.6 Dm existsbetween the average bubble diameter Dp and the average pore diameter Dm.

Example 3

The relationship for the minimum pressure ΔPc (critical pressure) atwhich bubble generation began for different average pore diameters inthe porous glass membrane was investigated in accordance with Example 1by varying the average pore diameter of the porous glass membrane. Theresults are shown in FIG. 5. The relationship between ΔP and Dm was inapproximate agreement with the equation shown above by (1) ΔP=4γ cosθ/Dm.

Example 4

The contact angle θ between the aqueous phase and the porous glassmembrane used in Example 1 was measured by the liquid-capillary-risingmethod (Yazawa, T., H. Nakamichi, H. Tanaka and K. Eguchi; “Permeationof Liquid through Porous Glass Membrane with Surface Modification,” J.Ceram. Soc. Japan, 96, 18-23 (1988)). The result was a contact angle ofθ=28°.

1. A method for producing bubbles by the injection and dispersion of agas through a porous body into a liquid, wherein the porous body has avalue of 1 to 1.5, wherein the value is given by dividing the porediameter that accounts for 10% of the total pore volume in the relativecumulative pore distribution curve of the porous body by the porediameter that accounts for 90% of the total pore volume in the relativecumulative pore diameter distribution curve of the porous body, whereinthe contact angle with respect to the liquid of at least the surface ofthe porous body that is in contact with the liquid is greater than 0°and less than 90°, wherein the gas is pressurized so that (1) thepressure is not less than the minimum pressure ΔPc given by thefollowing equation;ΔPc=4γ cos θ/Dm wherein γ is the surface tension of the liquid relativeto the gas, θ is the angle of contact relative to air of the liquidpresent at the surface of the porous body, and Dm is the average porediameter of the porous body, and (2) the pressure difference ΔP betweenthe pressure of the gas when the gas is pressured and the pressure ofthe liquid is controlled to 0.2 to 10 MPa.
 2. The method according toclaim 1, wherein porous glass is used as the porous body.
 3. The methodaccording to claim 1, wherein the liquid contains at least one additiveselected from the group consisting of emulsifying agents, emulsionstabilizers, foaming agents, and alcohols.
 4. Bubbles having the averagebubble diameter of 400 nm to 900 nm obtained by the method according toclaim
 1. 5. The bubbles according to claim 4, wherein, in the integratedvolume distribution of the bubbles, 1) the diameter at which the bubblevolume accounts for 10% of the total bubble volume is at least 0.5-timesthe diameter at which the bubble volume accounts for 50% of the totalbubble volume, and 2) the diameter at which the bubble volume accountsfor 90% of the total bubble volume is no more than 1.5-times thediameter at which the bubble volume accounts for 50% of the total bubblevolume.